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New X-ray, radio, and optical observations of the rich cluster Abell 2255 will be presented. These new observations have revealed interesting and unusual characteristics at each wavelength band that are inconsistent with a simple, relaxed cluster. Surprisingly, ROSAT all-sky survey (RASS) and CCD r-band images show that the cluster X-ray centroid does not coincide with any bright cluster galaxies or subgroup of galaxies. Furthermore, RASS spectra indicate cool (2 keV) gas within 5 arcmin of the X-ray centroid whereas previous X-ray satellites measured a temperature of 7 keV for the cluster as a whole. At 1240 km/s, A2255 has one of the highest known cluster velocity dispersions. In addition, the two brightest cluster galaxies are separated by an unusually large 2635 km/s. Our new VLA 20-cm imaging has revealed a distorted, off-center, steep-spectrum diffuse radio source, previously labeled a radio halo, and a peculiar ridge of radio emission about 0.6 Mpc north of the X-ray centroid. We believe that these cluster properties can be understood if A2255 is in the process of merging with a galaxy group. We will present a numerical Hydro/N-body simulation of such a merger which nicely reproduces the X-ray and galaxy velocity properties, and the merger has sufficient energy to power the diffuse cluster radio emission.
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